The invention relates to a method of reconstructing images from X-ray cone beam projection data, and also to a corresponding X-ray device.
It is known to use a C-arm X-ray system for imaging. First a set of X-ray cone beam projections is then acquired from an examination zone of an object to be examined. Subsequently, the two-dimensional or three-dimensional distribution of the X-ray attenuation coefficient within the examination zone to be imaged is reconstructed from such a set of cone beam projections. This distribution serves as a 2D image or 3D image of said examination zone.
The object to be examined is customarily a human body. The cone beam used for the projection is formed by a substantially punctiform X-ray source (the apex of the cone) and the sensitive surface area of the X-ray detector which can be reduced by switching over the electron-optical system. The examination zone to be imaged is then situated between the X-ray source and the detector. The X-ray source is attached to one end of the C-arm system and is guided along a predetermined trajectory around the examination zone to be imaged in order to acquire a set of cone beam projections. One cone beam projection after the other is measured at short intervals in time or space. Because of the described configuration, the trajectory is situated at least approximately on the surface of a sphere whose center constitutes the isocenter of the C-arm system.
When the trajectory is suitably chosen, a spherical volume can be imaged as the examination zone. The diameter of such a sphere is not very dependent on the choice of the trajectory, but much more on the dimensions of the detector and on some other geometrical parameters. In practice this diameter amounts to approximately 30 cm. Obviously, when the object to be examined is a human body, it will not fit in such a small sphere, which means that the cone beam projections are necessarily cut off and that the cone beam does not cover the entire body. The image of the examination zone, however, is unambiguously determined by all cone beam projections along the trajectory, that is, in as far as the projections are not cut off.
As a consequence of cut-off cone beam projections the image of the examination zone cannot always be unambiguously determined. This has a minor effect only inside the sphere. However, large variations may occur at the edge of the sphere. Such variations have a negative effect on the image quality, notably at the edge of the examination zone to be imaged. The reconstruction of cut-off cone beam projections will give rise to artefacts which degrade the image quality.
It is possible to switch over between different entrance field diameters in X-ray image intensifiers by changing the focusing voltage while maintaining the anode voltage. When the diameter of the entrance field is reduced, the resolution increases. On the other hand, as the resolution increases the examination zone that can be imaged is reduced, because the entrance field diameter is reduced. In other words, the negative effect which occurs when cone beam projections are cut off becomes more manifest as the resolution increases and the entrance field diameter is reduced accordingly.
Therefore, it is an object of the invention to provide a method and an X-ray device which enable the negative effect of the cut-off X-ray cone beam projections on the image quality to be reduced.
This object can be achieved in accordance with the invention by means of a method of reconstructing images from cone beam projection data of an examination zone of an object to be examined, the cone beam projection data being acquired by means of an X-ray device which includes an X-ray source and an X-ray image intensifier. The X-ray source is guided along a trajectory around the examination zone in order to acquire the projection data. The method includes the steps of: acquiring first projection data from the examination zone (3) in a first mode of operation of the X-ray image intensifier (2) with a low resolution; acquiring second projection data from a sub-zone of the examination zone in a second mode of operation of the X-ray image intensifier with a high resolution; combining the first and second projection data so as to form third projection data, the third projection data representing the second projection data in the sub-zone of the examination zone and the first projection data in the remaining part (3a) of the examination zone; and reconstructing images on the basis of the third projection data.
The above method can be carried out in accordance with the invention using an X-ray device which includes an X-ray source and an X-ray image intensifier for the acquisition of cone beam projection data from an examination zone of an object to be examined, the X-ray source being guided along a trajectory around the examination zone for the acquisition of the projection data, first projection data being acquired from the examination zone in a first mode of operation of the X-ray image intensifier with a low resolution while second projection data are acquired from a sub-zone of the examination zone in a second mode of operation of the X-ray image intensifier with a high resolution, and also includes means for combining the first and second projection data so as to form third projection data, the third projection data representing the second projection data in the sub-zone of the examination zone and the first projection data in the remaining part of the examination zone, and also includes an image processing device for the reconstruction of images on the basis of the third projection data.
The invention is based on the idea to acquire cone beam projection data of an examination zone of an object to be examined by means of an X-ray device which includes an X-ray source and an X-ray image intensifier, the X-ray source being guided along a trajectory around the examination zone in order to acquire the projection data. The X-ray image intensifier is first operative in a first mode which involves a low resolution. In this first mode of operation first projection data is acquired from the examination zone. Subsequently, the X-ray image intensifier is operative in a second mode of operation with a high resolution. In this second mode of operation second projection data is acquired from a sub-zone of the examination zone. Subsequently, the first and second projection data is combined so as to form third projection data. The third projection data is composed of the second projection data for the sub-zone of the examination zone as well as the first projection data for the remainder of the examination zone, that is, the difference zone between the examination zone and the sub-zone. Subsequently, the images are reconstructed on the basis of the third projection data.
This notably offers the advantage that the cone beam projections of the sub-zone are acquired with a high resolution, so that small details of the sub-zone are also visualized. The effect of the cut-off cone beam projections, notably at the edge of the sub-zone, is reduced, because in these locations recourse can be taken to the first projection data. In the first mode of operation of the X-ray image intensifier the occurrence of cut-off cone beam projections is far less in comparison with the second mode of operation, because in the first mode of operation essentially the entire examination zone is covered. The cone beam projections notably are not cut off around the sub-zone in the first mode of operation. Undesirable artefacts, notably in and around the sub-zone, can thus be avoided in the reconstruction.
In a version of the invention the number of first projection data, acquired in the first mode of operation, can be increased by 2D interpolation. This is advantageous because an increased number of first projection data results in an enhanced image quality of the reconstructed images, notably in the difference zone.
In a preferred version of the invention the sub-zone is masked by a diaphragm during the acquisition of the first projection data in the first mode of operation so that the X-ray dose is reduced. With a view to the fact that the second sub-zone is covered in a first as well as in a second mode of operation, it is thus achieved that the sub-zone is exposed to the X-rays only once, so that the overall X-ray dose is reduced.
In a further preferred version of the invention, the diaphragm is continuously readjusted during the acquisition of the first projection data in cases where the sub-zone is not centrally situated in the examination zone. The desired shielding of the sub-zone is thus ensured also when the sub-zone is shifted, relative to the X-ray source and the X-ray image intensifier, during the acquisition of the first projection data, while the X-ray source is guided along the trajectory around the examination zone for the acquisition of the projection data.
The invention also relates to an X-ray device as disclosed in claim 5 which may be configured and further elaborated in the same or similar way as the method described above and which may have corresponding advantageous embodiments.